Nope, that is it. You can even do a test checkout, and you will see the Amazon Payments as an option.

I wonder how widely used Amazon payments are.  Is there a market share that likely prefers this method?  Just finally added google checkout after someone suggested it.  It was easy!

Personally, I belive you will lose loaded shopping carts at checkout if you display the Amazon payments logo.  A customer might just decide they want to "shop around" some more on Amazon for the same items (because your checkout just "reminded" them about Amazon).  Your sales then can shift to your competitor and you are left with nothing other than an abandoned shopping cart. 

Payment systems are not what usually sell your products.  The sole exeption is Google Checkout which does send you customers from the Google search.  Amazon checkout does not send customers to your website.  Google does.  Amazon competes with your website.  Google does not.

Remember (except for Google Checkout) by the time the customer finds out about your payment system  they have already made a decision to purchase from you and have already picked out the products.  They are standing at the "cash register" ready to pay.  Don't let your customer get called out of the checkout line!

Joe